Prepare the coffee mixture: Brew strong coffee and let it cool to room temperature. Stir in coffee liqueur if using. Set aside.

Make the zabaglione: In a heatproof bowl, whisk egg yolks and sugar. Place over a pot of simmering water (double boiler) and whisk constantly until pale and thickened, about 10 minutes. Remove from heat.

Incorporate mascarpone: Let the egg mixture cool slightly, then gently fold in the mascarpone cheese until smooth and well combined.

Whip the cream: In a separate bowl, whip the heavy cream to stiff peaks. Gently fold the whipped cream into the mascarpone mixture.

Assemble the tiramisu: Quickly dip each ladyfinger into the coffee mixture (do not soak) and arrange in a single layer in a 9x13 inch dish.

Spread half of the mascarpone cream over the ladyfingers. Repeat with another layer of dipped ladyfingers and remaining cream.

Chill and serve: Cover and refrigerate for at least 4 hours, preferably overnight. Before serving, dust generously with cocoa powder.